DESCRIPTION (adapted from the applicant's description): Defects in septation and placement of pulmonary artery and aorta reflect diverse defects in a multifunctional pathway that controls modulation of the great vessels. Over the past two years, the PI's laboratory, based on study of homeodomain transcription factor interactions, discovered two bicoid-related homeodomain transcription factors, referred to as Pitx1 and Pitx2 that prove to be critical regulators of these events. The distribution of Pitx1 and Pitx2 expression strongly suggested that these might mediate distinct aspects of left/right asymmetry of the cardiovascular/pulmonary system, and specific aspects of subsequent cardiac development. Generation of gene-deleted mice has documented their critical function in both events. The Pitx proteins are positive transcription factors that act synergistically with other classes of transcription factors. They hypothesize that these genes are in a regulatory pathway required for aspects of correct septation and positioning of the developing aorta and pulmonary arteries, in part, by regulation of critical neural crest components. Pitx1 and Pitx2 also exert regulation in other aspects of cardiac development. The goal of this proposal is to investigate the molecular mechanisms by which Pitx1 and Pitx2 mediate these events, using several independent genetic approaches, to define their roles in normal cardiac development, their relationships with other factors, and their potential combinatorial roles in the development of the cardiac outflow tract. They propose to identify the downstream target genes that mediate their roles in cardiovascular development, thus, identifying potentially novel regulatory molecules that underlie congenital defects in outflow tract and coronary artery vessel development. Finally, they will identify and clone factors that interact with the Pitx transcription factors in the heart, and may be critically involved in the combinatorial transcriptional regulation of target genes critical for development of the truncus arteriosus. The possible role of environmental and regulatory factors in the animal model can therefore be studied, based on the modulation or alteration of these pathways.